In U.S. Pat. No. 4,895,629 of Jan. 23, 1990, Applicant has described a high-speed electrocoagulation printing method and apparatus in which use is made of a positive electrode in the form of a revolving cylinder having a passivated surface onto which dots of colored, coagulated colloid representative of an image are produced. These dots of colored, coagulated colloid are thereafter contacted with a substrate such as paper to cause transfer of the colored, coagulated colloid onto the substrate and thereby imprint the substrate with the image. As explained in this patent, the surface of the positive electrode is coated with a dispersion containing an olefinic substance and a metal oxide prior to electrical energization of the negative electrodes in order to weaken the adherence of the dots of coagulated colloid to the positive electrode and also to prevent an uncontrolled corrosion of the positive electrode. In addition, gas generated as a result of electrolysis upon energizing the negative electrodes is consumed by reaction with the olefinic substance so that there is no gas accumulation between the negative and positive electrodes.
The electrocoagulation printing ink which is injected into the gap defined between the positive and negative electrodes consists essentially of a liquid colloidal dispersion containing an electrolytically coagulable colloid, a dispersing medium, a soluble electrolyte and a coloring agent. Where the coloring agent used is a pigment, a dispersing agent is added for uniformly dispersing the pigment into the ink. After coagulation of the colloid, any remaining non-coagulated colloid is removed from the surface of the positive electrode, for example, by scraping the surface with a soft rubber squeegee, so as to fully uncover the colored, coagulated colloid which is thereafter transferred onto the substrate. The surface of the positive electrode is then cleaned to remove therefrom any remaining coagulated colloid.
The optical density of the dots of colored, coagulated colloid, hereinafter referred to as "pixels", may be varied by varying the voltage and/or pulse duration of the pulse-modulated signals applied to the negative electrodes. As a typical example, the printing head which carries the negative electrodes may comprise 2048 electrodes which are arranged to define 64 groups or channels each having 32 electrodes. By proper electronic circuitry, it is possible to sequentially scan the electrodes of each channel while performing such a scanning simultaneously for all channels, and to apply a pulse-modulated signal to selected ones of the electrodes during scanning to energize same. The pulse-modulated signal may have a pulse duration ranging from about 15 to about 4000 nanoseconds. An electrical signal with a pulse duration of 150 nanoseconds provides a pixel having an optical density of 0.02 (very light gray), whereas an electrical signal with a pulse duration of 4000 microseconds provides a pixel having an optical density of 1.50 (black). It is also possible to vary the pulse duration by a predetermined number of time increments, for example, 63 increments of about 60 nanoseconds each or 255 increments of about 15 nanoseconds each, depending upon the level of fidelity of reproduction required. A signal whose pulse duration can be varied from 15 to 4000 nanoseconds in 255 increments delivers of course the best tone reproduction. Thus, in this case, the printing of a pixel starts with a pulse duration of about 15 nanoseconds, progresses with 255 increments of about 15 nanoseconds up to 4000 nanoseconds and stops when the desired optical density is reached.
The negative electrodes are arranged in rectilinear alignment to define a series of corresponding negative electrode active surfaces which are disposed in a plane parallel to the rotation axis of the positive electrode and spaced from the surface thereof by a constant predetermined gap filled with the aforesaid electrocoagulation printing ink. Electrical energization of selected ones of the negative electrodes causes point-by-point selective coagulation and adherence of the colloid onto the olefin and metal-oxide coated positive electrode surface opposite the electrode active surfaces of the energized negative electrodes while the positive electrode is rotating, thereby forming the aforesaid dots of colored, coagulated colloid or pixels. The addressing mode of the negative electrodes is such that at any given time, a signal is impressed at a single electrode in each and every channel. In the example given above, at the beginning of the electrocoagulation printing, current injection is performed simultaneously through the 1st electrode of every channel; thus, 64 non-contiguous electrodes are energized at the same time. At the next cycle, the 2nd electrode in every channel is energized. This procedure is repeated until all the electrodes of the linear array have been energized.
Since the negative electrodes energized at any given point in time are non-contiguous and the film of electrocoagulation printing ink on the surface of the positive electrode constantly moves relative to the linear array of negative electrodes due to the rotation of the positive electrode, the electrode addressing mode creates a saw-toothed image resulting from the displacement of two adjacent pixels relative to one another along the direction of rotation of the positive electrode. Such a displacement is function of the time frame between the electrical energization of consecutive electrodes and also function of the speed of rotation of the positive electrode. The quality of the image thus reproduced is obviously less than perfect. Applicant has also observed the occurrence of overly dense pixels.